SiC (silicon carbide) is expected to serve as a material for a next-generation semiconductor device. SiC has more excellent physical properties in comparison with Si (silicon): band gap, three times; breakdown field strength, approximately ten times; and thermal conductivity, approximately three times. A low loss semiconductor device which can operate at a high temperature can be realized by using such properties.
However, for example, in the case where a metal insulator semiconductor (MIS) structure is formed by using SiC, since a breakdown voltage of SiC is high, a breakdown voltage of a gate insulating film may become lower than a breakdown voltage of a semiconductor. Especially, in the case where the MIS structure is formed in a trench to increase integrity of the device, there is a problem that a breakdown voltage of the gate insulating film is decreased by electric field concentration at a bottom of the trench.